1. Field of the Invention
The present invention relates generally to manufacturing processes requiring lithography, and more particularly, to measuring the foreshortening of array features that occurs during the lithographic process.
2. Description of Related Art
Lithography has a broad range of industrial applications, including the manufacture of semiconductors, flat-panel displays, micromachines, and disk heads.
The lithographic process allows for a mask or reticle pattern to be transferred via spatially modulated light (the aerial image) to a photoresist film on a substrate. Those segments of the absorbed aerial image, whose energy exceeds a threshold energy of chemical bonds in the photoactive component (PAC) of the photoresist material, create a latent image in the photoresist. In some photoresist systems the latent image is formed directly by the PAC. In others (so-called acid catalyzed photoresists), the photochemical interaction first generates acids which react with other photresist components during a post-exposure bake to form the latent image. In either case, the latent image marks the volume of photoresist material that either is removed during the development process (in the case of positive photoresist) or remains after development (in the case of negative photoresist) to create a three-dimensional pattern in the photoresist film.
The principal determinant of the photoresist image is the surface on which the exposure energy equals the photoresist threshold energy in the photoresist film. xe2x80x9cExposurexe2x80x9d and xe2x80x9cfocusxe2x80x9d are the variables that control the shape of this surface. Exposure, set by the illumination time and intensity, determines the average energy of the aerial image per unit area. Local variations in exposure can be caused by variations in substrate reflectivity and topography. Focus, set by the position of the photoresist film relative to the focal plane of the imaging system, determines the decrease in modulation relative to the in-focus image. Local variations in focus can be caused by variations in substrate film thickness and topography.
Generally, because of the variations in exposure and focus, patterns developed by lithographic processes must be continually monitored or measured to determine if the dimensions of the patterns are within acceptable range. The importance of such monitoring increases considerably as the resolution limit, which is usually defined as minimum features size resolvable, of the lithographic process is approached.
The foreshortening of array features (foreshortening is dimensional shrinking of a feature""s length, rather than it""s width) has been shown to be a highly sensitive measurement of lithographic focus, more so than feature width. Feature truncation is minimized at best focus. Currently, foreshortening measurements are performed using optical measurement systems. However, optical systems are susceptible to film variation, either in the imaging layer or underlying films, often requiring the optical system to be re-configured or re-calibrated to enable measurements.
An object of this invention is to provide a procedure for focusing lithographic processes.
Another object of the present invention is to provide an improved procedure for measuring the foreshortening of array features caused by lithographic processes.
These and other objectives are attained with a method and system for measuring lithographic image foreshortening. The method comprises the steps of providing a critical dimension scanning electron microscope, and using that critical dimension scanning electron microscope to measure lithographic image foreshortening. Preferably, a defined feature is formed using a lithographic process, and the critical dimension scanning electron microscope is used to measure foreshortening of that feature. For example, the feature may be a line, and the critical dimension scanning electron microscope may be used to measure foreshortening of the line. Also, the feature may be two arrays of lines, and the critical dimension scanning electron microscope may be used to measure the separation distance between the arrays. That separation distance may be used to determine a focus of the lithographic process.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description, given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.